Vacuum fluorescent displays are currently used as the display device in many common home digital clocks and the like. In addition to the common blue-green clock display, a variety of colored displays and high resolution graphic displays are offered as products.
The basic operation of a vacuum fluorescent display involves the emission of electrons from a low work function thermionic filament cathode, the acceleration of the thermionically emitted electrons by an electric field and stimulation of a phosphor anode by the impacting electrons resulting in cathodoluminescent light generation in the phosphor.
In the vacuum fluorescent display, the filament cathode to phosphor anode voltage determines whether the phosphor emits light and the intensity of the emitted light. The vacuum fluorescent display is similar to the common cathode ray tube found in televisions and monitors, except that the cathode ray tube uses electromagnetics to direct electrons to the addressed pixel while the vacuum fluorescent display requires that illuminated pixels be individually biased. Some vacuum fluorescent displays utilize mesh grids to "gate" the electron flow to cathode areas, thereby, the mesh grid may be used to select which pixels to light. By using an array of filament cathodes and biasing individual pixels in combination with grid structures, a matrix addressed flat panel display results. In contrast, the cathode ray tube requires a large depth to allow for electron beam deflection.
In vacuum fluorescent displays, typical filament cathodes are tungsten wire coated with barium and strontium compounds that allow sufficient electron emission at approximately 10 volts bias at 600.degree. C. filament temperature. The common phosphor used at this low bias voltage is ZnO:Zn, which gives a blue-green cathodoluminescent light emission.
A common 3/4" by 3" clock module draws approximately 300 mW to bias the filament (for heating). Further, because of the hot filaments the spacing between the hot filaments and the phosphor is substantial, currently the thickness is approximately 1/4".